Mussel populations show extensive hybridisation worldwide and are a well studied example of a marine hybrid zone. There are multiple sites of hybridisation between the species Mytilus edulis, M. trossulus and M. galloprovincialis across the Atlantic, Scandinavian and the Mediterranean Seas. These hybrid zones vary considerably. Some hybrid zones, such as the one in Newfoundland in Canada show remarkably few hybrids, while in Scandinavia most individuals are hybrids to some degree. Morphological and genetic differences are clear between these populations and it is believed that they are close to full speciation. They are probably maintained through a combination of hybrid unfitness, positive assortative mating and habitat segregation. In this summary focus will be on the Canadian hybrid zone in the North Atlantic, particularly that near Newfoundland which has been studied by researchers at Newfoundland Memorial University.
Based on the fossil record and genetic marker studies the following chronology is used to explain the Mussel hybrid zone:
The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Straight around 3.5 million years ago (Vermeij 1991).
M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Straight is very low.
Subsequently M. galloprovincialis undergoes cladogenesis from M. edulis in the Mediterranean Sea after it is temporarily isolated from the Atlantic.
Recently M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides. It spreads and forms secondary contact hybrid zones with M. edulis populations on coasts across Scandinavia and the eastern Atlantic.
Riginos and Cunningham (2005) includes a suggested pattern of migration of M. trossulus across the ocean based on a review of genetic marker studies.
The Canadian hybrid zone is unusual because both species are found along the entire shore (a mosaic pattern) instead of the typical cline found in most hybrid zones (Bates and Innes, 1995). Studies of mtDNA and allozymes in adult populations show that the distribution of genotypes between the two species is bimodal; pure parental types are most common (representing above 75% of individuals) while backcrosses close to parental forms are the next most prevalent. F1 hybrid crosses represent less than 2.5% of individuals (Saavedra et al., 1996).
The low frequency of F1 hybrids coupled with some introgression allows us to infer that although fertile hybrids can be produced, significant reproductive barriers exist and the two species are sufficiently deviated that they are now able to avoid recombinational collapse despite habitat sharing. Jiggins and Mallet (2000) have found that hybrid zones with low levels of F1 hybrids (bimodal distribution of hybrids) are highly stable and usually the result of assortative mating or fertilization, and not related to gross levels of genetic divergence or intrinsic genomic incompatibility. They hypothesized that such zones occurred between two populations that were close to full speciation and with some reinforcement. Toro et al. (2002) investigated whether different reproductive patterns and behaviour were the cause of this prezygotic isolation and discovered that M. edulis spawned over a 2-3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. It was also found that hybrids were not infertile and exhibited normal reproductive development, allowing them to introgress with pure species. It was concluded that “differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland.”
The other likely candidate for hybrid zones stability is species segregation by habitat which has been investigated but not conclusively. Several studies have suggested that M. edulis are found in areas of lower salinity and less wave exposure at the heads of bays more than M. trosullus. M.trosullus appears to be favoured in habitats with higher wave exposure (Bates and Innes, 1995). The one subtidal (low wave action) site sampled by Bates and Innes had just 8% M. trossulus individuals. A similar segregation has been found in the Mediterranean hybrid zone with M. edulis also favouring more sheltered habitats compared to M. galloprovincialis (Bierne et al. 2003). If this is the case, this would provide partial habitat separation and reduce the probability of gametes of two species encountering one another and cross-fertilising. This would increase genetic distinctiveness despite the populations living in sympatry. However, conflicting results have been identified to this trend of habitat segregation and so these results are not conclusive (Riginos and Cunningham, 2005). It is suggested that differences in habitat are what has led to the very different type of hybrid zones in Scandinavia and Canada. Hybrid mussel fitness has not been properly investigated, so it is not possible to judge its effects on postzygotic isolation and whether it could cause reinforcement
Based on the fossil record and genetic marker studies the following chronology is used to explain the Mussel hybrid zone:
The genus Mytilus is at one point restricted to the North Pacific but spreads to the Atlantic through the Bering Straight around 3.5 million years ago (Vermeij 1991).
M. trossulus evolves in the North Pacific and M. edulis in the Atlantic in near allopatry as migration across the Bering Straight is very low.
Subsequently M. galloprovincialis undergoes cladogenesis from M. edulis in the Mediterranean Sea after it is temporarily isolated from the Atlantic.
Recently M. trossulus from the Pacific enters the Atlantic and colonises shores on both sides. It spreads and forms secondary contact hybrid zones with M. edulis populations on coasts across Scandinavia and the eastern Atlantic.
Riginos and Cunningham (2005) includes a suggested pattern of migration of M. trossulus across the ocean based on a review of genetic marker studies.
The Canadian hybrid zone is unusual because both species are found along the entire shore (a mosaic pattern) instead of the typical cline found in most hybrid zones (Bates and Innes, 1995). Studies of mtDNA and allozymes in adult populations show that the distribution of genotypes between the two species is bimodal; pure parental types are most common (representing above 75% of individuals) while backcrosses close to parental forms are the next most prevalent. F1 hybrid crosses represent less than 2.5% of individuals (Saavedra et al., 1996).
The low frequency of F1 hybrids coupled with some introgression allows us to infer that although fertile hybrids can be produced, significant reproductive barriers exist and the two species are sufficiently deviated that they are now able to avoid recombinational collapse despite habitat sharing. Jiggins and Mallet (2000) have found that hybrid zones with low levels of F1 hybrids (bimodal distribution of hybrids) are highly stable and usually the result of assortative mating or fertilization, and not related to gross levels of genetic divergence or intrinsic genomic incompatibility. They hypothesized that such zones occurred between two populations that were close to full speciation and with some reinforcement. Toro et al. (2002) investigated whether different reproductive patterns and behaviour were the cause of this prezygotic isolation and discovered that M. edulis spawned over a 2-3 week period in July, while M. trossulus spawned over a more extensive period between late spring to early autumn. It was also found that hybrids were not infertile and exhibited normal reproductive development, allowing them to introgress with pure species. It was concluded that “differences in reproductive traits may partially explain the maintenance of the mussel hybrid zone in Newfoundland.”
The other likely candidate for hybrid zones stability is species segregation by habitat which has been investigated but not conclusively. Several studies have suggested that M. edulis are found in areas of lower salinity and less wave exposure at the heads of bays more than M. trosullus. M.trosullus appears to be favoured in habitats with higher wave exposure (Bates and Innes, 1995). The one subtidal (low wave action) site sampled by Bates and Innes had just 8% M. trossulus individuals. A similar segregation has been found in the Mediterranean hybrid zone with M. edulis also favouring more sheltered habitats compared to M. galloprovincialis (Bierne et al. 2003). If this is the case, this would provide partial habitat separation and reduce the probability of gametes of two species encountering one another and cross-fertilising. This would increase genetic distinctiveness despite the populations living in sympatry. However, conflicting results have been identified to this trend of habitat segregation and so these results are not conclusive (Riginos and Cunningham, 2005). It is suggested that differences in habitat are what has led to the very different type of hybrid zones in Scandinavia and Canada. Hybrid mussel fitness has not been properly investigated, so it is not possible to judge its effects on postzygotic isolation and whether it could cause reinforcement